[llvm-dev] Do I need to modify the AddrLoc of LLD for ARC target?

Peter Smith via llvm-dev llvm-dev at lists.llvm.org
Fri Sep 15 03:24:30 PDT 2017 

Hello Leslie,

If I understand you correctly, ld is aligning giving an Output or
Input Section alignment 1 and lld is giving an Output or Input Section
alignment 4 and you are trying to reduce the alignment in lld?

What I would do is try and work out why the Output or Input Section
has a higher alignment in lld. It is not a good idea to try and lower
the alignment of an OutputSection as this must have a high enough
alignment that every InputSection is aligned according to its
requirements, this means that at a minimum it must be set the maximum
alignment requested by an InputSection.

First think I would do would be to look at the linker map -M or
--Map=map.txt to put it in a file and look at the Output and
InputSections, if there is at least one InputSection that has
alignment 4 then I would expect the OutputSection to have alignment 4.
If all the InputSections in an OutputSection have alignment 1 I would
expect the OutputSection to have alignment 1 unless there is some
other bit of code enforcing a minimum alignment of 4.

In general overaligning an OutputSection is safe although the size of
the bytes added to align the Sections can get significant in a very
small embedded system.

Hope this is of some use.

Peter

On 15 September 2017 at 09:14, Leslie Zhai <lesliezhai at llvm.org.cn> wrote:
> Hi Peter,
>
> Thanks for your hint! it is the Alignment issue :)
>
> avr-ld and LLD both use the *same* Alignment 1 for
> input_section->output_section->vma, so VMA is same:
>
> DEBUG: avr-ld: R_AVR_CALL: VMA: 5 Output Offset: 1 Reloc Offset: 0
>
> DEBUG: avr-ld: R_AVR_8_LO8: VMA: 6 Output Offset: 0 Reloc Offset: 7
>
> ...
>
> DEBUG: lld: R_AVR_CALL TargetVA: 49 A: 44 P: 5 Align: 1 VMA: 5 Output
> Offset: 0 Reloc Offset: 0
>
> DEBUG: lld: R_AVR_8_LO8 TargetVA: 50 A: 44 P: 13 Align: 1 VMA: 6 Output
> Offset: 0 Reloc Offset: 7
>
> ...
>
> But arc-ld use Alignment 1 and LLD use *different* Alignment 4:
>
> alignTo
> https://github.com/llvm-mirror/llvm/blob/master/include/llvm/Support/MathExtras.h#L657
>
> alignTo(0x5, 1) = 5
>
> alignTo(0x5, 4) = 8
>
> alignTo(0x9, 1) = 9
>
> alignTo(0x9, 4) = 12
>
> alignTo(0x11, 1) = 17
>
> alignTo(0x11, 4) = 20
>
> ...
>
> I can *not* use updateAlignment to modify the Alignment for a smaller value,
> such as 1, it is monkey patch, not the root cause, so how to correctly
> modify the Alignment for LLD? please give me some hint, thanks a lot!
>
>
> PS: arc-ld's sym_section->output_section->vma is equals to LLD's
> SymbolBody::getVA, so the value of (S + A) is same.
>
>
> 在 2017年09月15日 12:33, Leslie Zhai 写道:
>>
>>
>>
>> 在 2017年09月14日 17:36, Peter Smith 写道:
>>>
>>> Hello Leslie,
>>>
>>> I think we are going to need to know a bit more about the ELF ABI for
>>> what looks like the ArcCompact before we can help you.
>>>
>>> LLD's calculation of P (the place to be relocated) is as it is in the
>>> generic ELF specification. The Rel.Offset corresponds to the ELF
>>> r_offset field. This is covered by: "For a relocatable file, the value
>>> is the byte offset from the beginning of the section to the storage
>>> unit affected by the relocation."
>>>
>>> For LLD we are calculating the virtual address (VA) of P, as I
>>> understand it this is equivalent to the vma used in BFD. Assuming that
>>> the relocation is relocating a regular InputSection from the
>>> basic-arc.o object then the LLD calculation of P =
>>> getOutputSection()->Addr + getOffset(Rel.Offset); translates to: (VA
>>> of OutputSection) + (Offset of InputSection within OutputSection) +
>>> (Offset within InputSection given by r_offset)
>>>
>>> The BFD linker seems to be doing the equivalent calculation with an
>>> extra modification of the (Offset within InputSection given by
>>> r_offset) and is rounding down the result to the nearest 4-byte
>>> boundary. This looks unfamiliar to me, and could well be specific to
>>> ArcCompact. I think that you will need to refer to the ELF ABI
>>> documentation as this should tell you if there are any processor
>>> specific modifications to generic ELF that you have to follow.
>>>
>>> The other thing that you should do is try and work out why the VA
>>> (vma) is 6 in LD and 8 in LLD and whether this is actually a problem.
>>> The VA of the OutputSection is not guaranteed to be the same between
>>> different linkers so it may have just been that differences in order
>>> of InputSections or alignment has caused a different VA. I would check
>>> the output of the linker map file to see where it placed the Output
>>> and Input Sections to see what the answer should be.
>>
>>
>> VMA in LD and LLD is the same for AVR target
>> https://reviews.llvm.org/D37615
>>
>> $ avr-gcc -mmcu=atmega328p -o basic-avr.o -c basic-avr.s
>> $ avr-ld -o basic-avr basic-avr.o -Ttext=6 /*with-different-high-address*/
>>
>> DEBUG: avr-ld: R_AVR_CALL: VMA: 6 Output Offset: 0 Reloc Offset: 0
>> DEBUG: avr-ld: R_AVR_16_PM: VMA: 6 Output Offset: 0 Reloc Offset: 4
>> DEBUG: avr-ld: R_AVR_8: VMA: 6 Output Offset: 0 Reloc Offset: 6
>> DEBUG: avr-ld: R_AVR_8_LO8: VMA: 6 Output Offset: 0 Reloc Offset: 7
>> DEBUG: avr-ld: R_AVR_8_HI8: VMA: 6 Output Offset: 0 Reloc Offset: 8
>> DEBUG: avr-ld: R_AVR_8_HLO8: VMA: 6 Output Offset: 0 Reloc Offset: 9
>> DEBUG: avr-ld: R_AVR_LO8_LDI: VMA: 6 Output Offset: 0 Reloc Offset: 10
>> DEBUG: avr-ld: R_AVR_HI8_LDI: VMA: 6 Output Offset: 0 Reloc Offset: 12
>> DEBUG: avr-ld: R_AVR_HH8_LDI: VMA: 6 Output Offset: 0 Reloc Offset: 14
>> DEBUG: avr-ld: R_AVR_MS8_LDI: VMA: 6 Output Offset: 0 Reloc Offset: 16
>> DEBUG: avr-ld: R_AVR_LDI: VMA: 6 Output Offset: 0 Reloc Offset: 18
>> DEBUG: avr-ld: R_AVR_LO8_LDI_NEG: VMA: 6 Output Offset: 0 Reloc Offset: 20
>> DEBUG: avr-ld: R_AVR_HI8_LDI_NEG: VMA: 6 Output Offset: 0 Reloc Offset: 22
>> DEBUG: avr-ld: R_AVR_HH8_LDI_NEG: VMA: 6 Output Offset: 0 Reloc Offset: 24
>> DEBUG: avr-ld: R_AVR_MS8_LDI_NEG: VMA: 6 Output Offset: 0 Reloc Offset: 26
>> DEBUG: avr-ld: R_AVR_LO8_LDI_PM: VMA: 6 Output Offset: 0 Reloc Offset: 28
>> DEBUG: avr-ld: R_AVR_HI8_LDI_PM: VMA: 6 Output Offset: 0 Reloc Offset: 30
>> DEBUG: avr-ld: R_AVR_HH8_LDI_PM: VMA: 6 Output Offset: 0 Reloc Offset: 32
>> DEBUG: avr-ld: R_AVR_LO8_LDI_PM_NEG: VMA: 6 Output Offset: 0 Reloc Offset:
>> 34
>> DEBUG: avr-ld: R_AVR_HI8_LDI_PM_NEG: VMA: 6 Output Offset: 0 Reloc Offset:
>> 36
>> DEBUG: avr-ld: R_AVR_HH8_LDI_PM_NEG: VMA: 6 Output Offset: 0 Reloc Offset:
>> 38
>> DEBUG: avr-ld: R_AVR_7_PCREL: VMA: 6 Output Offset: 0 Reloc Offset: 40
>> DEBUG: avr-ld: R_AVR_13_PCREL: VMA: 6 Output Offset: 0 Reloc Offset: 42
>> DEBUG: avr-ld: R_AVR_6: VMA: 6 Output Offset: 0 Reloc Offset: 52
>> DEBUG: avr-ld: R_AVR_16: VMA: 6 Output Offset: 0 Reloc Offset: 56
>> DEBUG: avr-ld: R_AVR_LO8_LDI_GS: VMA: 6 Output Offset: 0 Reloc Offset: 58
>> DEBUG: avr-ld: R_AVR_HI8_LDI_GS: VMA: 6 Output Offset: 0 Reloc Offset: 60
>> DEBUG: avr-ld: R_AVR_PORT6: VMA: 6 Output Offset: 0 Reloc Offset: 62
>> DEBUG: avr-ld: R_AVR_PORT5: VMA: 6 Output Offset: 0 Reloc Offset: 64
>> DEBUG: avr-ld: R_AVR_6_ADIW: VMA: 6 Output Offset: 0 Reloc Offset: 66
>>
>> $ llvm/build/bin/ld.lld -o basic-avr-lld basic-avr.o -Ttext=6
>> /*with-different-high-address*/
>>
>> DEBUG: lld: R_AVR_CALL TargetVA: 50 A: 44 P: 6 VMA: 6 Output Offset: 0
>> Reloc Offset: 0
>> DEBUG: lld: R_AVR_16_PM TargetVA: 50 A: 44 P: 10 VMA: 6 Output Offset: 0
>> Reloc Offset: 4
>> DEBUG: lld: R_AVR_8 TargetVA: 50 A: 44 P: 12 VMA: 6 Output Offset: 0 Reloc
>> Offset: 6
>> DEBUG: lld: R_AVR_8_LO8 TargetVA: 50 A: 44 P: 13 VMA: 6 Output Offset: 0
>> Reloc Offset: 7
>> DEBUG: lld: R_AVR_8_HI8 TargetVA: 50 A: 44 P: 14 VMA: 6 Output Offset: 0
>> Reloc Offset: 8
>> DEBUG: lld: R_AVR_8_HLO8 TargetVA: 50 A: 44 P: 15 VMA: 6 Output Offset: 0
>> Reloc Offset: 9
>> DEBUG: lld: R_AVR_LO8_LDI TargetVA: 50 A: 44 P: 16 VMA: 6 Output Offset: 0
>> Reloc Offset: 10
>> DEBUG: lld: R_AVR_HI8_LDI TargetVA: 50 A: 44 P: 18 VMA: 6 Output Offset: 0
>> Reloc Offset: 12
>> DEBUG: lld: R_AVR_HH8_LDI TargetVA: 50 A: 44 P: 20 VMA: 6 Output Offset: 0
>> Reloc Offset: 14
>> DEBUG: lld: R_AVR_MS8_LDI TargetVA: 50 A: 44 P: 22 VMA: 6 Output Offset: 0
>> Reloc Offset: 16
>> DEBUG: lld: R_AVR_LDI TargetVA: 50 A: 44 P: 24 VMA: 6 Output Offset: 0
>> Reloc Offset: 18
>> DEBUG: lld: R_AVR_LO8_LDI_NEG TargetVA: 50 A: 44 P: 26 VMA: 6 Output
>> Offset: 0 Reloc Offset: 20
>> DEBUG: lld: R_AVR_HI8_LDI_NEG TargetVA: 50 A: 44 P: 28 VMA: 6 Output
>> Offset: 0 Reloc Offset: 22
>> DEBUG: lld: R_AVR_HH8_LDI_NEG TargetVA: 50 A: 44 P: 30 VMA: 6 Output
>> Offset: 0 Reloc Offset: 24
>> DEBUG: lld: R_AVR_MS8_LDI_NEG TargetVA: 50 A: 44 P: 32 VMA: 6 Output
>> Offset: 0 Reloc Offset: 26
>> DEBUG: lld: R_AVR_LO8_LDI_PM TargetVA: 50 A: 44 P: 34 VMA: 6 Output
>> Offset: 0 Reloc Offset: 28
>> DEBUG: lld: R_AVR_HI8_LDI_PM TargetVA: 50 A: 44 P: 36 VMA: 6 Output
>> Offset: 0 Reloc Offset: 30
>> DEBUG: lld: R_AVR_HH8_LDI_PM TargetVA: 50 A: 44 P: 38 VMA: 6 Output
>> Offset: 0 Reloc Offset: 32
>> DEBUG: lld: R_AVR_LO8_LDI_PM_NEG TargetVA: 50 A: 44 P: 40 VMA: 6 Output
>> Offset: 0 Reloc Offset: 34
>> DEBUG: lld: R_AVR_HI8_LDI_PM_NEG TargetVA: 50 A: 44 P: 42 VMA: 6 Output
>> Offset: 0 Reloc Offset: 36
>> DEBUG: lld: R_AVR_HH8_LDI_PM_NEG TargetVA: 50 A: 44 P: 44 VMA: 6 Output
>> Offset: 0 Reloc Offset: 38
>> DEBUG: lld: R_AVR_7_PCREL TargetVA: 4 A: 44 P: 46 VMA: 6 Output Offset: 0
>> Reloc Offset: 40
>> DEBUG: lld: R_AVR_13_PCREL TargetVA: 2 A: 44 P: 48 VMA: 6 Output Offset: 0
>> Reloc Offset: 42
>> DEBUG: lld: R_AVR_6 TargetVA: 50 A: 44 P: 58 VMA: 6 Output Offset: 0 Reloc
>> Offset: 52
>> DEBUG: lld: R_AVR_16 TargetVA: 50 A: 44 P: 62 VMA: 6 Output Offset: 0
>> Reloc Offset: 56
>> DEBUG: lld: R_AVR_LO8_LDI_GS TargetVA: 50 A: 44 P: 64 VMA: 6 Output
>> Offset: 0 Reloc Offset: 58
>> DEBUG: lld: R_AVR_HI8_LDI_GS TargetVA: 50 A: 44 P: 66 VMA: 6 Output
>> Offset: 0 Reloc Offset: 60
>> DEBUG: lld: R_AVR_PORT6 TargetVA: 51 A: 45 P: 68 VMA: 6 Output Offset: 0
>> Reloc Offset: 62
>> DEBUG: lld: R_AVR_PORT5 TargetVA: 6 A: 0 P: 70 VMA: 6 Output Offset: 0
>> Reloc Offset: 64
>> DEBUG: lld: R_AVR_6_ADIW TargetVA: 50 A: 44 P: 72 VMA: 6 Output Offset: 0
>> Reloc Offset: 66
>>
>> interesting :)
>>
>>>
>>> In summary:
>>> It looks like there are some Arc specific things that might need to be
>>> done. Unfortunately I don't have any experience with Arc, and I'm not
>>> sure the other people that work on LLD do either. I suggest looking at
>>> the public ABI documentation and making any arguments for changes
>>> based on that documentation, it is worth assuming that we know nothing
>>> about Arc, don't have the documentation to hand and don't know where
>>> to find it!
>>>
>>> Hope that is of some help, with a bit more context I might be able to
>>> help a bit more, unfortunately I can't spend a lot of time learning
>>> about Arc.
>>>
>>> Peter
>>>
>>>
>>> On 14 September 2017 at 07:16, Leslie Zhai via llvm-dev
>>> <llvm-dev at lists.llvm.org> wrote:
>>>>
>>>> Hi LLVM developers,
>>>>
>>>> basic-arc.s:
>>>>
>>>> main:
>>>>    bl memset
>>>>
>>>> $ arc-elf32-gcc -mcpu=arc600 -o basic-arc.o -c
>>>>
>>>> $ arc-elf32-readelf -r basic-arc.o
>>>>
>>>> Relocation section '.rela.text' at offset 0xd4 contains 1 entries:
>>>>   Offset     Info    Type            Sym.Value  Sym. Name + Addend
>>>> 00000000  00000611 R_ARC_S25W_PCREL  00000000   memset + 0
>>>>
>>>> High address: 0x0
>>>>
>>>> $ arc-elf32-ld -o basic-arc basic-arc.o
>>>> -L/opt/arc-gnu/lib/gcc/arc-elf32/7.1.1/arc600
>>>> -L/opt/arc-gnu/lib/gcc/arc-elf32/7.1.1/../../../../arc-elf32/lib/arc600
>>>> -L/opt/arc-gnu/lib/gcc/arc-elf32/7.1.1
>>>> -L/opt/arc-gnu/lib/gcc/arc-elf32/7.1.1/../../../../arc-elf32/lib
>>>> --start-group -lgcc -lc -lnosys --end-group -Ttext=0
>>>>
>>>> DEBUG: arc-ld: R_ARC_S25W_PCREL relocation: 1 S: 4 A: 0 P: 0 = (vma: 0 +
>>>> output_offset: 0 + reloc_offset: 0 - 0) & ~0x3
>>>> DEBUG: arc-ld: type: R_ARC_S25W_PCREL insn: 2054
>>>>
>>>> $ ld.lld -o basic-arc-lld basic-arc.o $ARC_LINKER_LIB -Ttext=0
>>>>
>>>> DEBUG: lld: R_ARC_S25W_PCREL TargetVA: 4 A: 0 P: 0 <-- same P as arc-ld
>>>> DEBUG: lld: R_ARC_S25W_PCREL: Insn: 2050 Rel: 1
>>>> DEBUG: lld: R_ARC_S25W_PCREL: Insn: 2054 <-- same relocation value as
>>>> arc-ld
>>>>
>>>> But with several different high address *not* 0x0, such as 0x6:
>>>>
>>>> DEBUG: arc-ld: R_ARC_S25W_PCREL relocation: 2 S: 12 A: 0 P: 4 = (vma: 6
>>>> +
>>>> output_offset: 0 + reloc_offset: 0 - 0) & ~0x3
>>>> DEBUG: arc-ld: type: R_ARC_S25W_PCREL insn: 2058
>>>>
>>>> DEBUG: lld: R_ARC_S25W_PCREL TargetVA: 4 A: 0 P: 8 <-- different P?
>>>> DEBUG: lld: R_ARC_S25W_PCREL: Insn: 2050 Rel: 1
>>>> DEBUG: lld: R_ARC_S25W_PCREL: Insn: 2054 <-- different relocation value
>>>>
>>>> How arc-ld calculates P?
>>>>
>>>> P = ((reloc_data.input_section->output_section ?
>>>> reloc_data.input_section->output_section->vma : 0) +
>>>> reloc_data.input_section->output_offset + (reloc_data.reloc_offset -
>>>> (reloc_data.bitsize >= 32 ? 4 : 0))) & ~0x3;
>>>>
>>>> for example, R_ARC_S25W_PCREL's bitsize < 32, P = (6 + 0 + 0 - 0) & ~0x3
>>>> =
>>>> 4, when vma is 6, output and reloc offset is 0.
>>>>
>>>> How LLD calculates P (AddrLoc)?
>>>>
>>>> P = getOutputSection()->Addr + getOffset(Rel.Offset);
>>>>
>>>> for example, the same high address 0x6, LLD's P is 8, different with
>>>> arc-ld?
>>>> so do I need to modify the value of P for R_PC case in the
>>>> getRelocTargetVA?
>>>> please give me some hints, thanks a lot!
>>>>
>>>>
>>>> PS: arc-ld R_ARC_S25W_PCREL's FORMULA is: ( S + A ) - P ) >> 2, and it
>>>> needs
>>>> middle endian convert, so:
>>>>
>>>> Insn = middleEndianConvert (insn, TRUE);
>>>>
>>>> Insn = replaceDisp25w(Insn, ( S + A ) - P ) >> 2);
>>>>
>>>> Insn = middleEndianConvert (insn, TRUE);
>>>>
>>>> write32le(Loc, Insn);
>>>>
>>>> --
>>>> Regards,
>>>> Leslie Zhai - https://reviews.llvm.org/p/xiangzhai/
>>>>
>>>>
>>>>
>>>> _______________________________________________
>>>> LLVM Developers mailing list
>>>> llvm-dev at lists.llvm.org
>>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>
>>
>
> --
> Regards,
> Leslie Zhai -https://reviews.llvm.org/p/xiangzhai/
>
>
>

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